Heat insulation of buildings is well-known and there are numerous systems and products for insulating building surfaces, such as roofs and walls. The requirements to the insulation quality are constantly increasing which requires improved insulation systems and/or improved skills by the insulation installer. The requirements are mainly directed to improved insulation value which is often achieved by increasing the insulation thickness. Another issue is to avoid thermal bridges, so-called cold bridges, which often are caused by poorly installed insulation boards having gaps between them. A common way to meet these two requirements is to use a two-layer solution, wherein a first layer of insulation boards is first installed and then a second layer of insulation boards is installed on top of the first layer with the insulation boards of the second layer being shifted in relation to the insulation boards of the first layer.
If the building surface to the insulated is a flat, or substantially flat, roof it is often required that the installed insulation shall be able to support person traffic or even heavier traffic. This may be achieved by using a two-layer solution, wherein the first layer comprises heat insulation elements or boards and the second layer is a force distributing layer made of high density heat insulation boards or other materials.
A packing and/or transport unit with roof insulation elements for a two-layer solution is known from WO 2012/059192. The insulation elements include a number of lamellae and a few insulation boards, i.e. at least two different types. The insulation elements are used to insulate a flat roof construction. A predetermined number of transport units are provided on the roof. The elements of each of the transport units are laid out in two layers whereby an insulating layer is built on the roof. The two layers are typically made as a lower layer of lamellae and a top layer of larger insulation boards preferably having a higher density than the lower layer to provide a roof insulation which can carry load from e.g. building workers on the roof. The lamellae and the top layer boards are preferably provided in a staggered configuration. This solution is advantageous but does involve handling of many elements during the fitting of the roof insulation, which in turn is labour intensive and time consuming.
In another solution, which after installation looks like a two-layer solution, is to use so-called dual density insulation boards for flat roof insulation, wherein the two layers are included in a single product, namely an insulation board having a first layer of relatively low density and a second layer of higher density. An example of a dual density insulation board is known from e.g. WO 03/054264 A1.
A disadvantage with using the dual density products is though that when installing the dual density boards on the roof there is a risk that a gap will be provided between two adjoining boards creating a cold bridge from the roof to the roof membrane on top of the dual density boards. There is also a risk that the two adjoining dual density boards are slightly different in thickness and thereby the top surface of the insulation will not be at the same level thus creating one or more small steps from one dual density board to another. Such unevenness will be visible on the roof membrane provided as the exterior roof cover and does entail a risk of damage to the roof membranes, for instance as pools of rain water may form, or the like.